1. Field of the Invention
This invention relates to a functional element, in particular a cooktop or a control panel with a flat substrate of glass or glass ceramic in which a coating that contains a crosslinked polysiloxane is applied to the substrate.
2. Discussion of Related Art
Currently, cooktops can be manufactured in an extremely wide array of colors for a reasonable price if a plate of colorless glass, such as Borofloat®, soda-lime glass, or colorless glass ceramic is used as a substrate, whose underside, the side oriented away from the user, has a colored coating. Silicone-based coating systems have turned out to be particularly advantageous because they achieve the required imperviousness to heat conduction paste and the required color stability when exposed to thermal loading.
German Patent Reference DE 35 03 576 C2, European Patent Reference EP 1 267 593 B1, and Japanese Patent References JP H10-273 342 and JP2005 038 622 mention undercoatings in the form of two-layered systems of a pigmented glass layer serving as a first layer, directly on the underside of the substrate, and a silicone layer serving as a second layer.
Among other things, these systems have a disadvantage, as mentioned in Japanese Patent Reference JP2001 233 636, that the glass layer underneath significantly reduces the substrate strength.
Japanese Patent References JP2001 233 636, JP2001 233 637, JP2001 213 642 and JP2003 086 337 propose using among other things silicone resin as a bonding agent for the first layer, then applying a second layer to it, which is based on glass ceramic, aluminum titanate, or precious metal preparations, and firing both layers simultaneously at a temperature above 800° C. During the firing, the silicon decomposes completely, forming an inorganic silicon oxide network whose bond to the substrate is a weaker than in the above-mentioned glass-based systems so that there is no noticeable reduction in the strength of the substrate.
In order to eliminate the complex firing step, in which the inorganic layers of glass, aluminum titanate, precious metals, or glass ceramic are obtained at high temperatures (700-800° C.), Japanese Patent Reference JP2003 086 337 mentions a system composed of two layers of a heat-resistant resin. In it, the first layer contains effect pigments. So that the effect pigments can produce their iridescent effect, a second layer in which the heat-resistant resin is pigmented black and that serves as an opaque layer, is printed onto the back of the first layer. Both layers are hardened at only 200° C.
Single-layer silicone coating systems used as undercoatings for cooktops are also known.
German Patent Reference DE 25 06 931 C3 discloses a silicone-based enamel that can have aluminum, iron mica, and black pigments. The coating, however, is not used for affecting the color design of cooktops, but rather to increase the thermal conductivity of the cooktop in the infrared-heated heating range.
For undercoating and affecting the color design of cooktops Japanese Patent Reference JP2005 298 266 discloses a single-layer coating with a tri-functional methylphenylpolysiloxane as a bonding agent. The firing of the coating takes place at 200-350° C. The layer thickness is 10 μm and below. Because of the three-dimensional crosslinking of the silicone, the coating is impervious to heat conduction paste.
United States Patent Reference US2005 214 521 A1 describes single-layer silicone coatings for glass, and glass ceramic cooktops that remain color-stable when exposed to thermal loading of up to 700° C. The color stability in the temperature range from 20-700° C., despite the decomposition of silicone starting at 400° C., is supposedly achieved because the silicone resins used are largely free of organic groups.
For the successful use of silicone coatings as color bearing undercoatings of cooktops, in addition to color stability and opacity, properties such as imperviousness to heat conduction paste, adhesion strength, and scratch resistance are also important. In particular, a high level of scratch resistance of the silicone undercoating is a decisive factor in whether or not the cooktop can be transported and installed without being damaged. If the scratch resistance of the silicone coating is high, it is possible to reduce the number of defective plates produced and to simplify the installation of the cooktops. The consequently reduced number of rejects during production and installation makes it possible to achieve considerable cost savings.
Measurements of the scratch resistance of commercially available cooktops with silicone coatings, without the above-mentioned glass, glass ceramic, aluminum titanate, or precious metal layer, have shown that even with a load of 200-300 g, a rounded hard metal spike (radius: 500 μm) penetrates the coating and a scratch is detectable from the point of view of the user. It is immaterial here whether the silicone coating as described above is composed of two layers of a crosslinked silicone resin or of only one layer of an uncrosslinked, crosslinked, or pyrolized silicone resin. The hard metal spike removes the respective coatings down to the substrate with a load of 100 g, but no greater than 300 g.
Measurement of the temperature on the underside of cooktops has shown that with high-quality induction stovetops, but also in specially designed gas cooking appliances, in normal use, temperatures of only 150-250° C. are reached and in the worst-case scenario, an empty pot at maximum heat setting, a temperature of 350° C. is reached. By contrast with stovetops equipped with infrared-radiant heating elements, temperatures of up to greater than 600° C. are reached. The temperature regulation of induction stovetops essentially depends on the effectiveness of the installed electronic switching and regulating circuits and the position of the temperature sensors underneath the cooktop, such as taught by German Patent References DE 10 2005 031 392 A1, DE 10 2006 023 704 A1 and DE 196 46 826 C2. United States Patent Reference U.S. Pat. No. 3,742,174 discusses sophisticated technology that even permits cooktops to be made of plastic.